Printing

ABSTRACT

A method of printing comprising: storing a plurality of sets of linearization data ( 32 ) for a printhead ( 12; 14; 16; 18 ) with each set corresponding to a different history of usage of the printhead; monitoring (S 210 ) the history of usage of the printhead; selecting (S 220 ), prior to printing, from said plurality of sets of linearization data a set of linearization data that is the set that most closely matches the monitored history of usage of the printhead prior to said printing; and printing (S 250 ) the image using the selected set of linearization data.

A commonly used form of printer uses a moving printhead which scans fromside to side across a print medium so as to build up a printed image onthe medium. In this type of printer the print medium is generallystationary as the printhead is reciprocated back and forth. In this wayswaths of an image are printed on the medium with the print medium beingstepped after each swath. In contrast, in so-called “page wide array” or“full width” printers, fast printing can be achieved by using a fixedprinthead array which spans the full width of the area of the medium tobe printed. In such printers the print medium generally movescontinuously with respect to the stationary printheads during theprinting operation.

Printheads are typically manufactured in a manner that is in some wayssimilar to the manufacture of semiconductor integrated circuits. Fullwidth array printheads are difficult and costly to manufacture in oneunitary (“monolithic”) printhead. The print swath for a printhead isthus typically limited by the difficulty in producing very largesemiconductor chips or “die”. The failure of any one of a large numberof nozzles in a printhead can cause the loss of the entire full widthprinthead array. Because of this most full width array printheads areassembled from smaller subunits which can be individually tested priorto assembly into the full width printhead array.

Printhead dies invariable have different characteristics due tomanufacturing variability, and the size of the ink drops produced by theprintheads is variable within a manufacturing tolerance. For exampledifferent printhead die may print at slightly different densities. Forscanning printers a printhead is scanned across the page and the colourcontent of the page will generally appear fairly consistent. However,the need to complete numerous carriage passes back and forth across apage has meant that scanning inkjet printers are typically significantlyslower than other types of printer. On the other hand, page wide arrayprinters, which general have several printheads linearly spaced acrossthe print swath immediately adjacent to each other, have a comparativelyquick printing speed. However, since such printheads are immediatelyadjacent to each other defects and variability in the ink drop sizeproduced by the different printheads will be readily discernible,particularly when attempting to reproduce high quality graphics andimages. One type of colour uniformity difficulty produced by suchdefects/manufacturing tolerances is banding at the swath boundaries.

Embodiments of the invention are set out according to the appendedclaims.

An embodiment of the invention provides a method of printing comprising:storing a plurality of sets of linearization data for a printhead witheach set of data corresponding to a different degree of usage of theprinthead; monitoring the degree of usage of the printhead; selecting aset of linearization data, from said plurality of sets, that matches themonitored degree of usage; and printing using said selected set oflinearization data.

The degree of usage of the printhead may be determined from, forexample, one of:

(i) the length of time that the printhead has been idle before saidprinting;(ii) the amount of printed output the printhead has printed before saidprinting;(iii) the length of time that the printhead has been printing beforesaid printing; and(iv) any combination of (i) to (iii).

An embodiment of the invention provides a method of printing comprising:storing a plurality of sets of linearization data for a printhead witheach set corresponding to a different degree of usage of the printhead;monitoring the degree of usage of the printhead; and printing an imageas part of a print job comprising printing one or more images using aset of linearization data selected from said plurality of sets oflinearization data; wherein the selected set of linearization datacorresponds to a degree of usage which most closely matches themonitored degree of usage of the printhead prior to said printing.

A print job can be considered to be a unit of work to be run on aprinter and, for example, can comprise printing one or more files.

For the purposes of this specification the term “image” should beinterpreted broadly to mean any type of printed output.

An embodiment of the invention provides a printing system comprising:printing means for printing images; storage means for storing aplurality of correction factors; monitor means for monitoring thehistory of usage of the printing means; and processing means forselecting one or more correction factors from the storage means thatmatches the monitored history of usage and operating the printing meansusing said selected one or more correction factors.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings:

FIG. 1 illustrates a printing system according to an embodiment of theinvention;

FIG. 2 is a flow diagram that illustrates a calibration routineaccording to an embodiment of the invention;

FIG. 3 is a graph of luminosity across an ink target according to anembodiment of the invention;

FIG. 4 is a plot illustrating an example linearization data set for aprinthead according to an embodiment of the invention;

FIG. 5 illustrates, according to an embodiment of the invention, a plotof an optical property across a printed image for each of a plurality ofprints;

FIG. 6 is a flow diagram that illustrates a method printing according toan embodiment of the invention;

FIG. 6 a is a flow diagram that illustrates a method printing accordingto an embodiment of the invention;

FIG. 7 is a schematic illustration of a printhead assembly presented asbackground to an embodiment of the invention;

FIG. 8 is a graph showing the variation of optical density across twoadjacent printhead dies according to an embodiment of the invention; and

FIG. 9 is a flow diagram that illustrates a method of producinglinearization data according to an embodiment of the invention.

The optical density of the printed output produced by a printhead can,to an extent, depend on characteristics of the printhead. Thesecharacteristics can vary with time. For example the temperature of aprinthead generally changes during use. For example the printhead may berelatively cool at the start of a print job and heat up during thecourse of the print job. Such a temperature variation can manifestitself as changes in ink drop weight that can show up as non-uniformityin the colour of the printed output. That is, different prints of thesame image can have different appearances according to the degree ofusage of the printhead prior to the production of each particular print.As another example, when a printhead has been left idle (i.e. notprinting) for a period then the ink held in the printhead can have aconcentration gradient, for example, gravity may cause the ink towardthe bottom of the printhead to have a higher concentration then the inktoward the top of the printhead. This effect is sometimes referred to as“ink enrichment” and can be another source of colour non-uniformity.

To maintain the quality of the printed output the first few printsproduced by a prior art printhead during a printhead may be discardeduntil the printhead has achieved a steady state (e.g. at substantiallyconstant temperature). This is wasteful of ink, print media andoperating time. Some embodiments of the invention do not do this: theymay keep and use even the first few prints produced.

A service station may be mounted within a printer to clean the printer'sprintheads. During a service operation ink is fired through the nozzlesof the printhead in a process known in the printing arts as “spitting”.Spitting is used for clearing debris from the printhead nozzles.Servicing the printheads at regular intervals can also be used tomaintain colour uniformity, for example, spitting can be used to purgeink from the nozzle region of the printhead after the printhead has beenstored or otherwise not been used for a long period of time (forexample, to remove enriched ink from the vicinity of the nozzles).Servicing is therefore also wasteful of ink and operating time.Additionally, the ink collected from the spitting process will requiredisposal. Some embodiments of the present invention can be serviced lessoften than prior art printheads and still get good results.

To produce the range of tonal values required for graphics andphotographs, printheads are typically calibrated, or “linearized”, suchthat the print densities of halftone images substantially correspond tothe densities of the continuous tone, or “contone” images, which are tobe printed. Typically, measurements of the actual print density of theprinthead are made over the range of print densities and a curve fittingroutine then linearizes the data. The linearization data simply maps aset of input values to a new set of output values. The linearizationdata may be stored in a memory as a look-up-table (LUT) or ascoefficients of an equation.

FIG. 1 illustrates a printing system comprising a printhead assembly 10which is controlled by a processor 20 to print onto a print medium 60.The processor 20 has access to a memory 30 which stores linearizationdata 32 which is used as part of the print pipeline when printing animage. In an embodiment of the invention the linearization data 32 isgenerated by measuring an optical property of printed images produced bythe printhead arrangement 10 using an optical sensor 50. The memory 30may also contain pre-built linearization data, such as linearizationtables, which can be updated according to measurements made by theoptical sensor 50.

Some or all of the various components of the printing system may behoused together with the printhead assembly 10 in a printer or may belocated remotely from the printhead assembly 10, for example as part ofa computer system that is in wired or wireless connection with theprinthead assembly 10. Some of the components may be configured tocommunicate with each via a local network (eg Ethernet) and/or via theInternet. For example the memory 30 and/or the processor 20 may be partof the printer (i.e. “on-board”) or they may be remote from the printer.The memory and/or the processor may comprise a single memory/processoror may comprise a plurality of memories/processors. In some cases partof the memory and/or processor is on-board the printer whilst part ofthe memory and/or processor is remote from the printer.

A calibration routine may be performed for producing and/or updating thelinearization data 32 held in the memory 30. The calibration routine maybe performed before the printer is deployed in the field, for example,in a factory following the manufacture of the printer or otherwise priorto the sale or deployment of the printer. The calibration routine couldalso be performed in the field (either in addition to, or instead of,pre-deployment calibration).

The optical sensor may make measurements of optical density or may makemeasurements of luminosity, L*, (as will be discussed later) or someother optical property (eg reflectance, transmittance, absorbance). Theoptical sensor 50 could be an integral part of the printer (eg as aso-called “on-board” sensor) or it may be a separate system that isconnected to the printer when calibration is required. For example, theoptical sensor 50 may take the form of a high resolution instrument thatis more suited to use in a factory setting than as an integral componentof the printer that is actually sold/deployed. The optical sensor 50 maybe suitable to form an integral/embedded part of the printer. In someembodiments the printer that is sold or otherwise deployed does not havean embedded optical sensor and calibration is performed either in thefactory (i.e. offsite) or an optical sensor is connected to the printerto perform the calibration (eg as part of a servicing schedule or whenthe printheads or ink supply are changed). In some embodiments theoptical sensor 50 may take the form of a densitometer.

The printhead assembly 10 may comprise one or more printheads. Inherentmanufacturing imperfections and environmental conditions can affect theaccuracy of printing within the same model or make of printhead andgenerally, when the printhead assembly comprises a plurality ofprintheads, linearization data is generated for each individualprinthead.

FIG. 2 is a flow diagram that illustrates a calibration routine S100according to one particular embodiment of the invention.

At step S110 the calibration routine is initiated. The calibrationroutine could be initiated by user activation (say by pressing a buttonor a touch sensitive screen or by sending a message to the printer via acomputer that is in communication with the printer). The calibrationroutine may also be initiated automatically, for example the calibrationroutine could be initiated whenever the printer is switched on,instructed to perform a print job, or has the printheads or ink supplychanged. The calibration routine could also be preformed periodically,eg at set times or after set amounts of usage (which could be measuredas an amount of time printing or as an amount or print produced (egnumber of printed sheets produced)).

At step S120 a plurality of images are printed. The images may take theform of test patches or “targets”. The targets may take the form of a“ramp” of print densities deposited on the print medium 60. By way ofexample, the ramp may include regions printed with sixteen discreteprint densities. Print density is a measure of how much ink has beendeposited on to an ink medium. It is a logarithmic scale with a densityof 0.00 indicating that 100% of the light falling on the sample of printmedium is being reflected, a density of 1.00 indicates that only 10% ofthe incident light falling on the sample is being reflected and adensity of 2.00 indicates that only 10% of the incident light falling onthe sample is being reflected etc.

Each image corresponds to a different degree of usage of the, or each,printhead in the printhead assembly 10. For example, the images could beprinted consecutively so that each image corresponds to an increment tothe amount of time that a printhead has been operating. In anotherexample one or more images could be printed after different periods oftime that a particular printhead has remained idle and/or stored.

The sensitivity of the appearance of the printed output to differenthistories of usage may be determined experimentally to ascertainappropriate periods at which to produce the printed images (targets). Inone scenario the output from a printhead may depend on the time sincethe printhead was last used over a comparatively short time-scale (egover a few seconds, tens of seconds, a few minutes or tens of minutes)due to the effect of the printhead cooling down since it was last used.In another scenario the output from a printhead may depend on the timesince the printhead was last used over a comparatively large time-scale(eg over an hour, a few hours, a day or several days or longer) due tothe ink enrichment effects (eg the ink settling toward the bottom of theprinthead). In yet another scenario the removal of enriched ink from theprinthead during the first few prints would cause a variation in theprinted output over a short time-scale.

For a constant rate of printing (eg as measured in sheets/pages per unittime, images per unit time or whatever) then a certain time of printingwill be correspond to certain amount of printing.

At step S130 optical measurements are made, using the optical sensor 50,on each of the plurality of images. The optical measurements may providevalues of L* in a colour metric space known as CIE L*a*b* colour space(“CIELAB”) established by the Commission International de l'Eclairage.CIELAB was developed to take into account the variation of the humaneye's response to light across the range of visible wavelengths (i.e.colours). L* or luminosity is a measurement of optical density orlightness, with 0 being defined as pure black and 100 pure white. b* isa measurement along the yellow-blue coordinate. Negative a* is green andpositive a* is red.

FIG. 3 is a graph with L* as a function of the tone value, in the range0 to 255, for an ink target printed on the print medium 60. Generallythe ink target will be produced using a single ink colour, for exampleit may be one of black, cyan, yellow or magenta ink. The values of L*may, for example, be derived from measurements of optical density (eg byusing a densitometer) or by making measurements of L* directly.

At step S140 linearization data is generated based on the opticalmeasurements, for example, from the L* values. FIG. 4 is a plotdepicting an exemplary linearization table for a printhead die. The plotshows the input level on the horizontal axis and the output level on thevertical axis. The response curve represents the calibration/correctionthat is necessary to cause the printhead to print tones linearly in theprinter colour space. For example, the input level may be an 8-bit valuefrom 0 to 255, which is converted to a linearized output level, also an8 bit value from 0 to 255. The curve may be stored in the memory 30 ascoefficients of a mathematical formula or as a table of values 32.

At step S150 the linearization data generated for each image isassociated with the history of usage of the printhead when that imagewas produced. At step S160 the linearization data and the associatedhistory of usage is stored in the printer's memory 30.

It should be appreciated that embodiments of the invention are notnecessarily limited to the particular sequence of steps illustrated inFIG. 2 nor are all the illustrated steps necessarily essential. Forexample, rather than first printing a plurality of images (S120) thenmaking optical measurements on these images (S130) an alternative wouldbe to make the optical measurements and generate a set of linearizationdata for a particular printed image before proceeding to print anotherimage from the plurality of images.

FIG. 5 illustrates, for each of a number of consecutive prints (Print 1,Print 2, . . . Print n), a graph that shows the variation of opticaldensity or other optical property across the printed image. The graphsshown in FIG. 5 may be produced by scanning the optical sensor 50 acrossrespective prints. Alternatively, the graphs may be produced withoutscanning, for example by using an optical sensor array or CCD camerathat covers the full width (or a significant proportion of the fullwidth) of the printed image. The measured optical property of an imagecan be used to produce a set of linearization data (eg in the form of aLUT) that can be associated with the history of usage of the printheadwhen that image is printed. It can be seen from FIG. 5 that the graphsproduced from the first few prints show a large variation between eachother. This variation may be due to, for example, the effect oftemperature as the printhead heats up or by enriched ink being used up.For later images (eg images 5 and 6 and subsequent images in FIG. 5)there is little variation between the graphs, for example because theprinthead has reached a steady-state temperature. It can be seen thatthe same linearization data may be used to print images after a certainnumber of images have been printed (eg after five prints using theillustration of FIG. 5).

In some embodiments, rather than generate a completely new set oflinearization data for different images and different histories of usagea correction factor or set of correction factors may be applied to areference set of linearization data to achieve a set of linearizationthat is relevant for the image to be printed. For example after runninga calibration routine it may become apparent that the first few imagesthat a printed are darker than the images printed when the printheadreaches a steady state. This may be due to, for example, ink enrichmenteffects that have caused a higher concentration of ink to settle nearthe printhead nozzles. A correction factor or factors may be applied tothe set of linearization data used for steady state printing to producea set of linearization data that can be used to make the appearance ofthe first few prints more consistent with the appearance of the steadystate prints.

FIG. 6 is a flow diagram illustrating a method of printing according toa particular embodiment of the invention.

At step S200 the printing process is started when the printer receives aprint job. The print job may contain one or more images to be printed.

At step S210 the degree of printer usage is determined before an imageis printed. The printer usage may be monitored by a usage monitor 40(illustrated in FIG. 1). The degree of printer usage may be determinedin several different ways for example by determining any one of: theidle time of the printer before the print job commences; the amount ofprinting performed in the print job before the image is printed; theamount of printing performed in one or more previous print jobs beforesaid image is printed; the length of time that the printer has beenprinting in the current print job, and/or previous print jobs, beforesaid image is printed; and any combination of the aforementionedmethods.

At step S220 a set of linearization data is retrieved from the memory 30which has associated usage history data that most closely matches theprinter usage history determined in step S210. Then, at step S230, animage is printed using the retrieved set of linearization data.

At step 240 a check is made as to whether printing is complete, that isthat all the images in the print job have been printed. If the printingis complete the printing process stops at step S250.

At step S260, if the printing is not complete, the linearization dataused to print the last image in step S230 is compared to thelinearization data used to print the image or images previous to thelast image. If the linearization data has not converged then theprocessing continues to step S210 where the printhead usage is againdetermined and the next image is printed using the linearizationappropriate for current state of printhead usage for the next image.This processing repeats until either the linearization data hasconverged or the print job has finished.

At step S260, if the linearization data has converged then the samelinearization data can be used to print subsequent prints at step S270until printing is complete at step S280 and the printing processingstops at step S290. The linearization data may be considered convergedwhen the linearization data sets for consecutive images are similarwithin a defined error or margin. The error/margin may be determinedsuch that linearization data sets when applied to the same input datawould produce an image that would give substantially the same appearanceor such that the appearance of the two images would are acceptably closeto each other. The error/margin may be preset when the printer is madeor it may be a variable that a user may adjust. The convergence may bedetermined by comparing the linearization data of a printed image fromthe image printed immediately previously or the two (or more) imagesprinted immediately previously to determine whether or not thelinearization data for all the compared images has converged.

It will be appreciated that some printing applications may requiredifferent error/margins than others when determining convergence, forexample the production of high quality brochures may require a highdegree of convergence whilst the production of a draft document/work inprogress document may only require a lesser degree of convergence.

It should be appreciated that embodiments of the invention are notlimited to the particular sequence of steps illustrated in FIG. 6 norare all the steps necessarily essential. For example, as is illustratedin FIG. 6 a, the steps which correspond to the checking for theconvergence of the linearization data may be omitted.

FIG. 7 illustrates an example of a printhead assembly 10 that comprisesan array of multiple printheads 12, 14, 16, 18. Each printhead is shownhaving two linear arrays of print nozzles such as might be used to printtwo different ink colours. A printhead assembly 10 may includeprintheads for printing multiple ink colours or printing fluids, suchas, for example, cyan, magenta, yellow, black and fixer. The individualprintheads are generally staggered in a direction that is perpendicularto the direction of the media transport (indicated by arrows). Eachprinthead generally overlaps the span of adjacent dies by a smallamount. FIG. 8 illustrates an example of the optical density variationtypically found across two such adjacent printheads. The dotted lines onthe FIG. 8 represent the region of overlap of the two printheads.

The variation of the printed output of a printhead (or a number ofprintheads) can be corrected/ameliorated using the technique illustratedin the flow diagram of FIG. 9.

At step S310 the printhead is portioned into non-overlapping sectionsand a value of an optical property (for example the optical density orL*) is recorded for each section.

At step S320 one of the sections is nominated as a reference section.This allows the variation between a measured optical property of thereference section and the other sections to be quantified.

At step 330 an image is printed using the printhead. At step 340, foreach section, the variation between an optical property (eg opticaldensity) of the portion of the image corresponding to that section ofthe printhead with the portion of the image corresponding to thereference section of the printhead is determined.

At steps S350, S360 and S370, for each section, linearization data isgenerated by performing a transformation on linearization dataassociated with the reference section using the variation of themeasured optical property between the reference section and the othersections.

At step S380 the linearization data is stored and/or used to print animage.

At step S390 the processing is repeated for subsequent images in a printjob until the printhead reaches a steady state. Once the printhead is ina steady state the stored linearization data can be used for theremaining images.

It should be appreciated that embodiments of the invention are notlimited to the particular sequence of steps illustrated in FIG. 9 norshould all the steps be considered essential.

For a printhead arrangement comprising a plurality of printheads theprocessing illustrated in FIG. 9 can be performed for each printhead.Variations in the printheads due to manufacturing tolerance willgenerally mean that the linearization data for each printhead will bedifferent to each other. The printheads may also reach a steady-state atdifferent times, for example, because the printheads use different inks,the printheads have a different position in the printer (eg causing someprintheads to be hotter than others), and/or the printheads aredifferent to each other (eg due to manufacturing tolerances or being adifferent type/model/manufacturer).

In an embodiment of the invention a computer program may be used toconfigure a printer/printing system to perform as has been describedhereinabove. The computer program may be carried on a physical medium(eg on a floppy disc, CD, DVD, tape, EPROM, memory stick etc) or it maybe downloaded into the memory 30 of the printing system via acommunication link (eg the Internet, the Ethernet, or by some othernetwork).

An embodiment of the invention provides a method of manufacturing aprinter comprising: in factory/off site creation of linearization data(eg LUTs) for different print histories/usage and saving saidlinearization data in a memory; and loading a processor withinstructions to monitor printer usage in the field and to select thelinearization data from the memory which will most closely match thecurrent history of usage and to use that linearization data in aprinting operation.

In some embodiments the linearization data that is created “off site” isstored into a master memory and at least some of the linearization datais then copied from the master memory to a printer memory. Then, inorder to perform a printing operation, linearization data can beselected from the printer memory according to the history of usage ofthat printer.

Such a method may also comprise loading the processor with instructionsto enable the printer to check, from time to time, a printed image thathas been printed in the field, and to self-calibrate itself, knowing itsprint history at the time of printing the image, by altering itsinstructions to influence what linearization data is selected from itsmemory for future print operations.

An embodiment of the invention provides a method of ameliorating inkenrichment effects when a printer and/or printhead has stood idle, notprinting, and/or a method of ameliorating variations over time and/orspatial variations in printhead temperature, the method comprising:storing a plurality of sets of linearization data for a printer and/orprinthead with each set corresponding to a different history of usage ofthe printer and/or printhead; monitoring the degree of usage of theprinter and/or printhead; selecting, prior to printing the image, fromsaid plurality of sets of linearization data a set of linearization datathat is the set that most closely matches the monitored history of usageof the printer and/or printhead prior to said printing; and using theselected set of linearization data to print the image.

An embodiment of the invention provides a calibration routinecomprising: printing a plurality of images with each image being printedafter a different history of usage of a printhead; measuring an opticalproperty of each of the printed images; using said measured opticalproperty to associate different sets linearization data with respectivedifferent histories of usage; and storing the linearization data.

The calibration routine provides a method of establishing a set oflinearization data to be used with a printer.

It should be appreciated that embodiments of the invention describedand/or claimed in a particular category should also be taken to bedisclosed in other categories. For example it should be appreciated thatany particular printhead arrangement can be utilised in aprinter/printing system and that the printhead arrangement of aparticular printer/printing system may be made or sold separately fromthe printer/printing system. Similarly embodiments of the inventiondisclosed as methods can be realised as printers configured to performsuch methods and vice versa. Furthermore, embodiments of the inventiondescribed in terms of a printer controller should be taken to bedisclosed in terms of a printing system/method of printing and viceversa.

In some embodiments of the invention the history of a printer's usageprior to printing is used to determine the linearization data that isused in the printing. If the printer uses a plurality of printheads thenwhen the printer is idle all the printheads are idle and the history ofusage of the printer may be equally applicable to all the printheads. Inanother scenario a particular printhead, or group of printheads, mayhave recently been used to print a number of monochromatic images using,say, only black (K) ink. In this case the history of usage of theprinthead that delivers the black ink will be different to the historyof usage of the printer that delivers other colours of ink (eg C, Y, andM ink). Therefore, when printing a subsequent colour image, differentdegrees of usage for the different printheads should be considered whenretrieving the linearization data required for that image. Therefore,when embodiments of the invention are set out in terms of printer usagethen equivalent embodiments should be taken to be disclosed in terms ofprinthead usage and vice versa.

1. A method of printing comprising: storing a plurality of sets oflinearization data for a printhead with each set corresponding to adifferent history of usage of the printhead; monitoring the history ofusage of the printhead; selecting, prior to printing, from saidplurality of sets of linearization data a set of linearization data thatis the set that most closely matches the monitored history of usage ofthe printhead prior to said printing; and printing using the selectedset of linearization data.
 2. The method of claim 1 wherein saidprinting of an image is part of a print job and the history of usage isdetermined from one of: (i) the idle time of the printhead before theprint job commences; (ii) the amount of printing performed in the printjob before said image is printed; (iii) the amount of printing performedin one or more previous print jobs before said image is printed; (iv)the length of time that the printhead has been printing in the print jobbefore said image is printed; (v) the length of time the printhead hasbeen printing in one or more previous print jobs before said image isprinted; and (vi) any combination of (i) to (v).
 3. The method of claim1 comprising performing a calibration routine comprising: printing aplurality of images with each image being printed after a differenthistory of usage of the printhead; measuring an optical property of eachof the printed images; using said measured optical property to associatedifferent sets linearization data with respective different histories ofusage; and storing the linearization data.
 4. The method of claim 3,wherein said associating comprises one of: (i) selecting one or moresets of linearization data from said plurality of stored sets oflinearization data and associating said one or more selected sets oflinearization data with respective one or more images from the pluralityof printed images; (ii) updating one or more of said plurality of storedsets of linearization and associating said one or more updatedlinearization data sets with respective one or more images from saidplurality of printed images (iii) generating one or more new sets oflinearization data for one or more of the plurality of printed imagesand storing the new sets of linearization data; and (iv) any combinationof (i) to (iii).
 5. The method of claim 3 wherein the measured opticalproperty provides values of L* in L*a*b* colour space for the, or each,ink used to print the printed image and the method further comprisingusing said value of L* to generate said linearization data.
 6. Themethod of claim 4 comprising: (i) dividing the printhead intonon-overlapping sections; (ii) selecting one section as a referencesection and generating linearization data for that section; (iii)determining the variation between an optical property of a section of animage produced by the reference section of the printheads and a sectionof the image produced by one of the other sections of the printhead;(iv) producing the linearization data for the other section by mappingthe linearization data of the reference section based on the opticalproperty variation; and (v) repeating steps (iii) and (iv) untillinearization data has been produced for all the sections of theprinthead.
 7. The method of claim 6 wherein the optical property is L*in L*a*b* colour space.
 8. The method of claim 6 comprising generatinglinearization data for a portion of the printhead that corresponds to atransition between adjacent sections of the printhead by interpolatingbetween the linearization data of said adjacent sections of printhead.9. The method of claim 1 wherein each set of linearization datacomprises a look-up-table (LUT) or a set of coefficients of amathematical equation.
 10. The method of claim 1 wherein said printheadis one of a plurality of printheads and said printing comprises applyingthe method of claim 1 to each of the plurality of printheads.
 11. Aprinting system comprising: one or more printheads; a memory containinga plurality of sets of linearization data; a monitor configured tomonitor the degree of usage of the one or more printheads; a processorfor selecting a set of linearization data according to the degree ofusage of the one or more printheads obtained from the monitor and foroperating the one or more printheads with said selected set oflinearization data.
 12. The printing system of claim 11 wherein saidmonitor comprises a clock configured for one of: (i) timing the periodthat the one or more printheads are idle prior to operating saidprintheads; (ii) timing the period that the printing system is printingprior to said operating said printheads; (iii) and a combination of (i)and (ii).
 13. The printing system of claim 11 wherein said monitorcomprises a counter configured to count the number of images or sheetsof print printed by the printing system prior to said printing.
 14. Theprinting system of claim 11 wherein each set of linearization datacomprises a look-up-table (LUT) or a set of coefficients of amathematical equation.
 15. The printing system of claim 11, wherein thememory is pre-loaded with at least some of said plurality of sets oflinearization data.
 16. The printing system of claim 11, wherein theprocessor is configured to execute a calibration routine to perform anyone of: (i) generate said plurality of sets of linearization data storedin the memory; (ii) adjust at least some of said plurality of sets oflinearization data stored in the memory; (iii) supplement said pluralityof sets of linearization data stored in the memory with additionallinearization data; and (iv) any combination of (i) to (iii).
 17. Theprinting system of claim 16, wherein the processor is configured toexecute said calibration at instances chosen from any one of: (i) at thestart of a print job; (ii) after a defined number of prints; (iii) aftera defined period of printing; (iv) after a set idle time; (v) when oneor more of the printheads have been changed (vi) when the type of printmedium used by the printing system has been changed; and (vii) anycombination of (i) to (vi).
 18. A print controller having: a datastructure comprising sets of linearization data; a printhead usagememory; and a processor having access to the data structure and usagememory; wherein the processor is configured to use the data structureand usage memory to select and/or create linearization data to be usedfor a particular printing operation.
 19. A computer readable mediumcontaining program instructions for performing the method of claim 1.